1. Field of the Invention
Heretofore, suspension fertilizers produced by direct ammoniation of merchant-grade wet-process or other impure orthophosphoric acids in batch-type equipment were required to be of low grade in order to avoid the occurrence of high viscosities, which high viscosities tend to destroy the fluidity of such suspensions and render the transfer properties thereof either by gravity or pumping distribution to the soil impractical or impossible. Suspension fertilizers with such low analysis have a very distinct economic disadvantage as compared with higher analysis products because costs of handling, freight, storage, and application are higher per unit of plant nutrient.
2. Description of the Prior Art
The art of producing fluid fertilizers, both liquids and suspensions, is well known and fully described in the literature. See, for example, the following patents:
______________________________________ U.S. Pat. No. Author Date ______________________________________ 3,015,552 Striplin, et al January 2, 1962 3,019,099 Walters January 30, 1962 3,109,729 Slack, et al November 5, 1963 3,113,858 Slack, et al December 10, 1963 3,148,970 Smith, et al September 15, 1964 3,192,013 Young June 29, 1965 3,234,004 Smith, et al February 8, 1966 3,326,666 Walters June 20, 1967 3,382,059 Getsinger May 7, 1968 3,459,499 Mullen August 5, 1969 3,697,247 Jones, et al October 10, 1972 3,813,233 Kendrick May 28, 1974 4,066,432 Jones January 3, 1978 4,375,980 Jones, et al March 8, 1983 ______________________________________
The most common method of producing both liquid and suspension fertilizers containing nitrogen and phosphate is by the ammoniation of phosphoric acids or slurries of solid products derived therefrom, such as, for example, monoammonium phosphate. For many years, all ammonium phosphate fluid fertilizers with both high analyses and good physical properties contained substantial proportions of the P.sub.2 O.sub.5 values therein in the form of polyphosphates. Polyphosphates increased the solubility of the phosphate salts and sequestered congeneric metallic impurity compounds present, which salts and compounds othewise would contribute to the formation of gel-like compounds that completely destroy the fluidity of concentrated suspension fertilizers. However, only in recent years was suitable technology developed for the production, from impure phosphoric acids, of satisfactory concentrated orthophosphate suspension fertilizers by methods which overcome the effect caused by the formation of the metallic impurity gel-like compounds, which compounds normally tend to destroy the fluidity of concentrated ammonium orthophosphate suspension fertilizers. Until the development of the latest technology, the only way known for increasing the fluidity of such orthophosphate suspensions was by means of dilution of same with water, which means, of course, reduced the grades thereof.
In 1962, Walters ('099 supra) produced a low-analysis (8-24-0 grade) ammonium orthophosphate suspension fertilizer by both batch- and continuous-type ammoniation of wet-process orthophosphoric acids. In the preparation of this suspension fertilizer product, Walters recognized that the impurities in the acid imparted a thixotropic characteristic causing higher grade suspension fertilizers to be excessively viscous. Walters used the gel-like impurity compounds for prevention of sedimentation in his suspension fertilizer products. Later Slack et al ('929 and '858, supra) produced NPK suspension fertilizers in which they added both polyphosphate for increasing the grade, and clay as both a nucleating and suspending agent. In 1974 Kendrick ('233 supra) fully described the techniques and conditions under which ammonia can be reacted with impure wet-process orthophosphoric acid without producing the gel-like impurity compounds that previously completely destroyed the fluidity of high-grade orthophosphate suspension products. Kendrick's teachings were based primarily on ammoniation of the impure wet-process acid in two continuous stages or steps and through the techniques he developed, he was able to produce much higher grade products (e.g., 11-39 -0 vs. 10-30-0) with good flow characteristics and long-term static storage life than heretofore had been possible. Kendrick also produced similar products by a batch-simultaneous procedure which he indicated did not work quite as well as did his continuous process. However, when products that were produced as taught by Kendrick were shipped by rail, the solid portion of the suspensions settled and packed. The resulting packed crystals held tenaciously to the bottom of the railroad tank cars. Settling of the crystals changed the composition of the fluid and the packed crystals could not be removed from the tank by ordinary procedures, thereby greatly reducing the value of the suspension fertilizer. In 1977 Jones ('432 supra) developed a three-stage, continuous-type process for the production of satisfactory high-analysis suspension fertilizers (e.g. 13-38-0) by the ammoniation of wet-process orthophosphoric acid under conditions wherein gel-like impurity compounds were not formed. In the materials produced by the teachings of Jones, supra, the resulting crystals did not settle and pack due to the application thereto of vibrational energy such as that which occurs during shipment by rail. However, until the present time, all efforts to produce high-analysis ammonium orthophosphate (N-P.sub.2 O.sub.5) base suspensions (e.g., 12-36-0, 1.5 percent clay, to 13-38-0, 1.5 percent clay) with satisfactory physical properties by the ammoniation of impure wet-process orthophosphoric acid in simple, economical, batch-type equipment instead of the more expensive and more complicated continuous-type equipment described by Jones in '432 supra, have been unsuccessful.